A modern emulsion formula
Ok, I hear you.
From the posts here, e-mails and PMs, people don't care whether I'm the major poster here, or whether they are making emulsions or not or whether anyone else is either. They just want to learn how emulsions are made.
I liken it to learning how to call the fire department in case of an emergency. "Hello, APUG, I want to look at an emulsion formula. Yes, my favorite product was just discontinued." - Just kidding, but a real possibility, so here goes with the modifications necessary to turn the 'old' formula into a 'modern' formula. THIS IS NOT A REAL FORMULA! It is merely what I would do as a first cut to turn the old formula into a modern one, as a leaping off place for experiments. I cannot make the final formula at home. You will see why. The intermediate formulas, showing evolutionary trends are also very difficult.
Here is the original formula posted before:
Potassium Bromide 132 grams
Potassium Iodide 4.5 grams
Gelatin 30 grams
Water 1 liter
Silver nitrate 130 grams
Water 500 ml
Heat A and B to 45 deg C
Add 28% ammonium hydroxide to B with stirring until a clear solution results.
TURN OUT LIGHTS!
Add B -> A over 10 minutes
Hold for 30 minutes at 45 deg C.
Let stand for 2 hours or until at room temperature.
Shred into noodles and wash. (make sure all salts and ammonia are removed)
Remelt and adjust gelatin percent to the desired level (5 - 10%)
Add spectral sensitizing dye and hold at 45 deg for 15 mins.
Coat with a hardener and surfactant.
First, the ammonia above is used to 'churn' the iodide in the crystal to spread it evenly over the entire emulsion rather than place it in the core. Here is one way to fix that problem.
Add an equal amount of ammonium sulfate to the gelatin and salt solution and leave it out of the silver nitrate solution. Make the emulsion as described above adding B -> A. At the end of the 10 minute addition, add enough dilute sodium hydroxide to the emulsion to release the ammonia. Up to that point, the ammonia was inactive, being slightly acidic. I would then hold for an exact amount of time and then add acid to neutralize the ammonia in the emulsion. This gives precise control over the ammonia digest step and improves uniformity.
In a variation on the above, or step 2, you can leave out the iodide, and inject a portion of the silver nitrate. Hold for about 2' and then dump in a solution of the iodide in gelatin. Then the additional salt and silver is run in.
That was step 3 in the evolution of emulsions leading to precise emulsion formulas. Now we go to step 4. This is the most modern method of emulsion making.
1. Solution A is now salt and gelatin, but an amount of salt is removed equal to silver. It is placed in a solution C. It is only sodium bromide. Iodide is not added to either A or C, but rather is in solution D.
So, you have A, which is a little sodium bromide and all of the gelatin, C which is sodium bromide equal to the silver nitrate and in the same amount of water. You have D which is the potassium iodide in water equal to the silver nitrate.
The amount of water used in A is decreased by the amount of water used in C and D.
Solution B is basically the same as above. It is silver nitrate alone.
Turn out the lights.
Add about 1/10th of solutions B + C + D -> A for about 10" at exactly equal rates.
Hold for about 2'
Add gelatin in water to equal the final desired level.
Add the remainder of B + C + D -> A while controlling the pAG (negative log of the silver ion concentration) at an exact value and ramping the flow rate using the equation: Flow = a + bt + ct^2. This equation is a polynomial which is determined to fit the growth of the surface area of the emulsion. The factors a, b and c are determined by trial and error, and t = time.
This stage is used to add the metal salts or organic chemicals that control latent image keeping, reciprocity and raw stock keeping so that the chemicals are evenly distributed in the crystals, but there are many many methods.
Adjust final pAg by adding salt or silver.
Now, you can see that this emulsion is far more complex than the previous ones. It is impossible to make without computer control.
So, you see some of the steps in emulsion technology evolution.
Modern emulsions take hours to make with fully automated computer driven equipment that follows a set formula with little human intervention. Many different salt and silver concentrations are actually used and often dozens of chemicals are added during the make. Sometimes there are up to 25 discrete steps.
Following the wash step, the finising or chemical sensitization with sulfur compounds is carried out. This area is very confidential and I will not go into it at all. However, it is fully as complex as the making.